The reason it's so bad for Arms is that Slam holds the swing timer, making Haste much worse than you'd think. Let's say you use Slam as your Rage dump (as we are expected to, confirmed by the big Crab) after stacking a bunch of haste to get more Auto Attacks in. Your weapon speed is now a crazy 2.0 (down from a 3.5, super haste stacking FTW). If you hit Slam your swing is now 2.5 (.5sec cast added for each Slam). That means that you just wasted .5 seconds worth of Haste.

Mastery on the other hand is never devalued by Slam. Something to note here is the misconception that SoO only procs off auto attacks, it procs off any MELEE attack. As all but 3 of our attacks are melee (Heroic Throw, Shattering Throw, and Thunderclap) it can (and does) proc from Slam and other attacks, so throwing that extra Slam in there doesn't devalue the stat, in increases it instead.

Now, there is a third group of Arms Warriors out there and they say, "To hell with Slam, I'm using Heroic Strike instead!" They are reporting that HS gives better DPR and DPS than Slam does on live and on the beta. These Warriors may find Haste much more valuable than normal, as Slam isn't messing with their swing timer and the extra attack speed brings the rage in faster. HOWEVER, GC himself put out a statement on the Forums that pretty much says if Arms is choosing HS over Slam then we may need to buff one or nerf the other (link at bottom of post). I haven't tried this out but then again my guild hasn't raided ICC since 4.0 hit. This would be the only way that Haste would be a good stat for Arms unless they remove the Swing Pause from Slam.

As for stats, after the hit and expertise caps are reached, crit should be around 30% unbuffed then stack strength and mastery. Avoid Haste if you can, and Reforge Haste to useful stats, like Mastery or Crit, if you can't.

http://blue.mmo-champion.com/topic/130295/arms-rage-concerns-pve